As mobile communication and portable electronic devices are closely related to our real life, a charged battery in case of emergency can cause serious trouble to users. In this regard, necessity of a portable or auxiliary power feeding apparatus used in the mobile communication or portable electronic device is increasing.
As the emergency power feeding apparatus, a piezoelectric power generator can suggest a solution. When an external mechanical energy (wind, instrumental vibrations, etc.) is applied, the piezoelectric power generator produces power using a piezoelectric effect inducing polarization (electric energy) inside a material.
Meanwhile, a piezoelectric body which is a core material of the piezoelectric power generator includes three basic modes based on three elements of a polarization direction, a poling direction, and a piezoelectric body deformation direction. The basic modes are a 31 mode, a 33 mode, and a 15 mode. In each mode, the former number indicates the electrical polarization direction and the latter number indicates the mechanical displacement direction.
For example, it is assumed that an electrode is spread over an upper part and a lower part in every piezoelectric body (an electrical field direction of every piezoelectric body is perpendicular to the ground). In this case, the 31 mode generates the electrical polarization direction perpendicular to the mechanical displacement direction, the 33 mode generates the same electrical polarization direction as the mechanical displacement direction, and the 15 mode generates a shear strain (sliding) with the electrical polarization direction in parallel with the mechanical displacement direction.
FIG. 1 is a schematic diagram of a 15-mode piezoelectric body. In FIG. 1, A1 indicates the 15-mode piezoelectric body before the deformation and A2 indicates the 15-mode piezoelectric body after the deformation.
Piezoelectric power energy is affected by a displacement constant of the piezoelectric body, a piezoelectric voltage constant, and the applied force. Typically, in the piezoelectric material, the 31 mode has the smallest piezoelectric voltage constant and the 15 mode has the greatest piezoelectric voltage constant. Without considering the shape, the greatest energy value can be obtained in the 15-mode piezoelectric body. Yet, it is difficult to manufacture the 15-mode piezoelectric body because the shear strain is required.
On account of those shortcomings, a conventional piezoelectric power generator mainly employs the 31-mode piezoelectric body which is relatively easy to implement. However, since the 15-mode piezoelectric body exhibits the power generation ten times greater than the 31-mode piezoelectric body, the piezoelectric power generator using the 15-mode piezoelectric body is demanded.